The present invention relates to a method for producing a flow which is rich in methane and a cut which is rich in C2+ hydrocarbons from a flow of dehydrated feed natural gas, the method being of the type comprising the following steps of:                cooling the feed natural gas flow advantageously at a pressure greater than 40 bar in a first heat exchanger and introducing the cooled, feed natural gas flow into a first separation flask;        separating the cooled natural gas flow in the first separation flask and recovering a light fraction which is substantially gaseous and a heavy fraction which is substantially liquid;        dividing the light fraction into a flow for supplying to a turbine and a secondary flow;        dynamic expansion of the turbine supply flow in a first expansion turbine and introducing the expanded flow into an intermediate portion of a separation column;        cooling the secondary flow in a second heat exchanger and introducing the cooled secondary flow into an upper portion of the separation column;        expanding of the heavy fraction, vaporisation in the first heat exchanger and introduction into a second separation flask in order to form a head fraction and a bottom fraction;        introducing the head fraction, after cooling in the second heat exchanger, in the upper portion of the separation column;        introducing the bottom fraction into an intermediate portion of the separation column;        recovering, at the bottom of the separation column, a bottom flow which is rich in C2+ hydrocarbons and which is intended to form the cut rich in C2+ hydrocarbons;        removing, at the head of the separation column, a head flow rich in methane;        reheating the head flow rich in methane in the second heat exchanger and in the first heat exchanger and compressing that flow in at least a first compressor which is connected to the first expansion turbine and in a second compressor in order to form a flow rich in methane from the compressed head flow rich in methane;        removing a first recirculation flow from the head flow rich in methane; and        passing the first recirculation flow into the first heat exchanger and into the second heat exchanger in order to cool it, then introducing at least a first portion of the first cooled recirculation flow into the upper portion of the separation column.        
Such a method is intended to be used to construct new units for producing a flow which is rich in methane and a cut of C2+ hydrocarbons from a feed natural gas, or in order to modify existing units, in particular when the feed natural gas has a high content of ethane, propane and butane.
Such a method is also used when it is difficult to carry out cooling of the feed natural gas by means of an external cooling cycle using propane, or when the installation of such a cycle would be too expensive or too dangerous, as in, for example, floating plants or in built-up regions.
Such a method is particularly advantageous when the unit for fractionating the cut of C2+ hydrocarbons which produces the propane which is intended to be used in the cooling cycles is too far from the unit for recovering that cut of C2+ hydrocarbons.
Separating the cut of C2+ hydrocarbons from a natural gas extracted from underground allows economic imperatives and technical imperatives alike to be satisfied.
Indeed, the cut of C2+ hydrocarbons recovered from the natural gas is advantageously used to produce ethane and liquids which constitute raw petrochemical materials. It is further possible to produce, from a cut of C2+ hydrocarbons, cuts of C5+ hydrocarbons which are used in oil refineries. All these products can be exploited economically and contribute to the profitability of the installation.
Technically, the demands placed on natural gas supplied commercially via networks include, in some cases, a specification in terms of the calorific power which must be relatively low.
Methods for producing a cut of C2+ hydrocarbons generally comprise a distillation step, after the feed natural gas has been cooled, in order to form a head flow which is rich in methane and a bottom flow which is rich in C2+ hydrocarbons.
In order to improve the selectivity of the method, it is known to remove a portion of the flow rich in methane produced at the column head, after compression, and to reintroduce it, after cooling, at the column head, in order to constitute a reflux of this column. Such a method is described, for example, in US2008/0190136 or in U.S. Pat. No. 6,578,379.
Such methods allow recovery of ethane to be obtained that is greater than 95% and, in the latter case, even greater than 99%.
However, such a method is not completely satisfactory when the feed natural gas is very rich in heavy hydrocarbons and in particular ethane, propane and butane, and when the introduction temperature of the feed natural gas is relatively high.
In such cases, the quantity of cooling to be provided is high, which requires the addition of a supplementary cooling cycle if it is desirable to maintain good selectivity. Such a cycle consumes energy. In some installations, in particular floating installations, it is further not possible to implement such cooling cycles.